Vinyl chloride, or monochloroethylene (CH.sub.2 .dbd.CHCl), has been known since the early nineteenth century. With the growth of polyvinyl chloride polymers (PVC), vinyl chloride as the basic "starting" material became more commonly referred to as vinyl chloride monomer (VCM), and has become a product of extremely important commercial significance. During 1976, almost five billion pounds of PVC were produced in the United States alone.
VCM has been commercially synthesized by various combinations of processes and procedures, but these generally fall into one of two basic routes: (1) the hydrochlorination of acetylene, or (2) the oxyhydrochlorination (OHC) and/or chlorination of ethylene to 1,2-dichloroethane (EDC), followed by a pyrolysis reaction in which the dichloroethane is pyrolyzed to VCM and hydrogen chloride. Except where the context otherwise clearly indicates, the terms chlorinating, chlorination, oxychlorination and/or oxyhydrochlorination reaction, etc., shall be understood to denote any of the various procedures for producing EDC from ethylene.
While the hydrochlorination of acetylene is obviously the easier and more direct route chemically, acetylene is a significantly higher-priced hydrocarbon than ethylene. On the other hand, the economic advantage of ethylene as a starting material is, in part, offset by the more complex series of reactions required and also by the energy imbalance which is inherent in these reactions when conducted separately.
The ethylene route involves first an exothermic chlorination reaction most frequently followed by an endothermic pyrolysis reaction. In terms of the total calories of heat produced, the heat generated by such an OHC reaction is two to three times greater than the heat input required for the pyrolysis reaction. In spite of this, it has heretofore been impossible, in commercial operation, to conduct the pyrolysis reaction employing only the heat from the OHC reaction, because the OHC reaction is usually carried out at a temperature about 200.degree. C lower than the temperature of the pyrolysis reactor. In essence, while the quantity of heat generated may be sufficient, it is not heat of a sufficient "quality," (that is, a high enough temperature) and additional energy input has heretofore generally been required for the pyrolysis reaction.
It is obvious, of course, that the ideal solution would be a process in which heat generated in the chlorination reaction(s) would be of sufficient quality, as well as quantity, to conduct the pyrolysis reaction; such as by lowering the temperature at which the pyrolysis reaction can be conducted and/or raising the temperature of the chlorination reactor, while maintaining substantially the same efficiency in conversion of ethylene to VCM. While such a process will be referred to hereinafter as an "adiabatic process," it will be clear that it does not require heat balance, but merely the elimination of the need for any substantial independent input of higher temperature heat.
In its most desirable form, such an adiabatic process would compromise chlorination of ethylene to dichloroethane in an exothermic OHC reaction, which would provide the heat for substantially simultaneous, highly VCM selective, in situ pyrolysis of the dichloroethane to VCM and hydrogen chloride. Since the HCl by-product of the pyrolysis reaction could be consumed in situ for the OHC reaction, such a process would eliminate not only the need to collect the heat output of the chlorination reaction and the need to transfer that heat to a pyrolysis reactor, but it would also eliminate or substantially reduce the need to recover, purify and recycle most of the hydrogen chloride by-product.
A "simultaneous reaction" process is, in fact, taught in British Pat. No. 1,159,296. However, the specific catalyst used in the teaching of this British patent is not very highly selective to VCM, and the resultant overall low percentage yield of ethylene to VCM greatly reduces the potential commercial significance. In addition, this process requires that significant volumes of hydrogen chloride be fed to the reaction necessitating recovery and recycle of this raw material at considerable operating expense.
A process having a more desirable ethylene to VCM yield is taught in U.S. Pat. No. 3,291,846 (Otsuka et al.) where a gaseous mixture of ethylene, chlorine and ethylene dichloride are fed at a temperature of from 450.degree. to 550.degree. C to a first reactor (a fluidized bed containing only sand) to form a product containing VCM, HCl and unreacted ethylene from which the VCM is removed, leaving only hydrogen chloride and ethylene, to which oxygen is then added. The mixture of ethlyene, oxygen, and hydrogen chloride is then fed to a second reactor, where it is catalytically oxyhydrochlorinated to a mixture of ethylene dichloride and chlorinated by-products, with the ethylene dichloride being purified, particularly of oxygen, and recycled to the first reactor. In essence, this process comprises a non-catalytic vapor phase chlorination of a stoichiometric excess of ethylene, at a temperature in excess of that at which EDC would normally be pyrolyzed to VCM, and in which the reaction equilibrium is further directed towards the production of VCM by employing a feed stream in which the ratio of carbon to chlorine is greater than 2:1. As with most non-catalytic reactions to yield a reactive intermediate, this procedure can be expected to produce large quantities of by-product chlorohydrocarbons.
The detailed steps of a balanced process involve much more than simply achieving an energy balance or elimination of the need for independent heat input to the pyrolysis reaction. It should also provide a high degree of selectivity to VCM, while minimizing by-products by shifting the reaction equilibrium either away from this production or to a form in which they can be recycled to the original reactor.
For example, since the energy output of the OHC reaction is about twice the energy input required for the pyrolysis reaction, the most desirable feed to the reactor is not just an oxygen and chlorine source, plus ethylene, but an oxygen and chlorine source, plus a mixture of ethylene and EDC. This provides not only energy balance, but also materials balance in that the HCl, ethylene, oxygen, and unpyrolyzed EDC can be recycled to the original reactor either directly, or after a separate OHC reaction in which the HCl and ethylene are converted to EDC.
There are a wide variety of catalytic oxyhydrochlorination processes well known to those skilled in the art, most frequently employing cupric chloride (alone or with a modifier), impregnated on an alumina, silica, or other support. While this is, perhaps, the most widely employed catalyst for oxyhydrochlorination, other chlorides, including iron chlorides, have been employed. A wide variety of materials have been employed with the cupric chloride to modify one or more of the characteristics of the catalyst. Potassium chloride is probably the most frequently employed modifying material, usually added to reduce the evaporation losses of cupric chloride, though other alkali metal chlorides have been similarly employed.
For example, in the aforementioned British Pat. No. 1,159,296, the catalyst employed was copper chloride and potassium chloride on a diatomaceous earth support. As previously noted, the selectivity to VCM exhibited by this catalyst is low. In addition, while the catalyst was reported to be suitable for use in either fixed or fluid bed reactors, the specification indicates that only the fixed bed reactor could provide operation under adiabatic conditions.